Which "local languages" are top priority and why?

Is there a difference in the likelyhood for volunteering translators to quit depending on the method?

More professionals equals fewer to no volunteer translators on those particular language, I would guess.

Would not the bounty rallies and pay per message methods be open to anyone to benefit from?

Even pay per message would require some commitment from the translator, IMNSHO, with regards to time and volume to keep things manageable.

Rallies are likely to be less and less effective [..]. How do you solve this in your bounty rallies?

I don't as long as the results are still satisfying; I partially steer the results in the rules by demanding some groups to be translated before others, and in the currently running rally by adding a bonus to the two top contributors. If the results would be expected too low in the current form, or if the latest rally does not render sufficient result, I would think up a way to get better results next time - basically train and error.

Is this not considered a problem because you have to have a translator privileges to be able to translate?

We give out translator privileges very easily and assume good faith until proven otherwise; it works quite well, indeed. When in doubt, we seek confirmation or another translator for the same language will notify staff, and we will take corrective measures.

Would the less "easy" messages be left untranslated?

See previous thoughts on steering and commitments in time and volume, but it is a threat, though minor.

Possibilities for customising the site?

Everything is possible; I would however request some WMF developer help and support on getting it done, as the WMF already has a lot of infrastructure in this area.

Thanks alot for the information!

I drafted a reccomendation based on this at Localisation, please take a look at it as I reffer some estimating to you.